1. Field of the Invention
The present invention relates to a focusing control system for performing a focusing control in an optical disc apparatus which records or reproduces information on or from an information carrier having a large number of information tracks using a beam emitted from a semiconductor laser diode or the like.
2. Description of the Prior Art
FIG. 1 schematically depicts a conventional focusing control system provided in an optical disc apparatus. An information carrier or recording medium 1 is mounted on a rotary shaft of a motor 2 for rotation together therewith at a predetermined speed. A spiral groove having a number of tracks, on which signals have already been recorded, is formed on the recording medium 1. The width and the pitch of the tracks are 0.6 .mu.m and 1.6 .mu.m, respectively. A beam 4 emitted from a light source 3 such as, for example, a semiconductor laser diode is collimated by a coupling lens 5 and then passes through a polarization beam splitter 6 and a 1/4 wavelength plate 7. Thereafter, the parallel beam is reflected from a total reflection mirror 8, converged by an objective lens 10, and is eventually irradiated on the recording medium 1. The objective lens 10 is secured to a movable member 40 having a bearing. The bearing allows the movable member 40 to move up and down along a vertically extending slide shaft 51 secured to a carrier 13 and to rotate about the slide shaft 51. The movable member 40 is connected with the carrier 13 by means of a rubber spring 52 and is appropriately adjusted such that an optical axis of the beam 4 is generally aligned with that of the objective lens 10. The carrier 13 can move on a platform 53 in a direction radially of the recording medium 1. When a coil 89 is energized by making an electric current flow therein, the movable member 40 receives the electromagnetic force therefrom and moves up and down along the slide shaft 51.
Light 9 reflected from the recording medium 1 passes through the objective lens 10 and is further reflected from the total reflection mirror 8. Thereafter, the light passes through the 1/4 wavelength plate 7 and is reflected from the polarization beam splitter 6. The light then enters a detection lens 81 and passes therethrough. At this moment, a part of the light is irradiated on an optical detector 11, whereas the remaining part of the light is reflected from a reflection mirror 82 and is used for a trucking control. The optical detector 11 is of two-split construction, and two outputs therefrom are input into respective amplifiers 16 and 17. Outputs from the two amplifiers 16 and 17 are input to respective input terminals of a differential amplifier 18, which in turn outputs a signal corresponding to a difference between two input signals.
This method is generally known as a "knife-edge method". The output from the differential amplifier 18 is a focusing error signal indicating a deviation between the position of a point (this point is hereinafter referred to as a focus of the beam 4) on which is focused the beam 4 converged by the objective lens 10 and the position of a recording surface of the recording medium 1. The focusing error signal is hereinafter referred to as an FE signal. The FE signal is input to an A/D converter 30 and to an in-focus detection circuit 50. The A/D converter 30 converts input analogue signals to digital signals and outputs the digital signals. The output from the A/D converter 30 is input to a D/A converter 31 via a phase compensation digital filter 22, which provides the focusing control system with a phase margin. The D/A converter 31 converts input digital signals to analogue signals and outputs the analogue signals. The output from the D/A converter 31 is input to a drive circuit 35 via a switch 33. The drive circuit 35 causes an electric current corresponding to an input signal thereof to flow in the coil 89.
As described above, the electric current corresponding to the FE signal is caused to flow in the coil 89 and moves the objective lens 10 up and down, thereby achieving an appropriate focusing control operation so that the beam 4 is always focused on the recording surface of the recording medium 1. The switch 33 has three input terminals A, B, and C, one output terminal D, and one control terminal E. When the control terminal E is at a low level, the input terminal A is connected to the output terminal D. When the control terminal E is at a middle level, the input terminal B is connected to the output terminal D. When the control terminal E is at a high level, the input terminal C is connected to the output terminal D.
In general, the "knife-edge" method can detect the deviation in the range of about 20 .mu.m. If the deviation between the focus of the beam 4 converged by the objective lens 10 and the recording surface of the recording medium 1 is out of this range, the focusing control cannot be appropriately performed even when the input terminal A and the output terminal D of the switch 33 are connected to each other. Because of this, the deviation between the focus of the beam 4 and the recording surface of the recording medium 1 is rendered to fall within said range by moving the objective lens 10 in advance upon application of the electric current to the coil 89. Under such conditions, when the input terminal A and the output terminal D of the switch 33 are connected to each other, the focusing control can be performed appropriately.
The input terminal C of the switch 33 is connected to a triangular wave generator 41, which generates a signal required to move the objective lens 10. The switch 33 switches between a condition wherein the deviation between the focus of the beam 4 and the recording surface of the recording medium 1 is rendered to fall within the detectable range by moving the objective lens 10, and a condition wherein the focusing control is in operation. The operation for making the deviation between the focus of the beam 4 and the recording surface of the recording medium 1 fall within the detectable range by moving the objective lens 10 is hereinafter referred to as an initial operation. During the initial operation required for the focusing control, when the in-focus detection circuit 50 detects that the position of the focus of the beam 4 converged by the objective lens 10 is generally in agreement with that of the recording surface of the recording medium 1, the in-focus detection circuit 50 sends an in-focus detection signal to a microcomputer 37. The initial operation is triggered at the time the microcomputer 37 connects the input terminal C and the output terminal D of the switch 33 and sends a detection start instruction to the in-focus detection circuit 50 with a data line 70 rendered to be at a high level. When the in-focus detection signal is sent from the in-focus detection circuit 50 to the microcomputer 37, the microcomputer 37 connects the input terminal A and the output terminal D of the switch 33 to allow the focusing control operation.
In general, as the accuracy of quantization in the A/D converter 30 is reduced, control errors increase. Because of this, it is necessary to increase the number of bits in the A/D converter 30 in order to perform an A/D conversion in the full range of the FE signal and to keep the control accuracy required during data reproduction or the like, i.e., the accuracy of quantization. This, however, causes an increase in manufacturing cost of the A/D converter 30. Accordingly, the accuracy of quantization required for appropriate data reproduction or the like is generally maintained by restricting the range of the FE signal to be A/D-converted without increasing the number of bits.
In such a conventional focusing control system, the level of the FE signal is increased by a transient response at the time the focusing control operation is started or by a disturbance caused by, for example, some impact during the focusing control operation. As a result, the problem arises that the FE signal may exceed an A/D convertible level below which it can be A/D-converted. In particular, if the recording surface of a recording medium undulates during rotation thereof, an increase in level of the FE signal caused by the transient response is relatively large, and the FE signal appreciably exceeds the A/D convertible level. Under such conditions, although the level of the FE signal is increased, the level is made constant after the A/D conversion, thus making the focusing control system unstable. At the worst, the distance between the focus of the beam and the recording surface of the recording medium may exceed the detectable range of the FE signal. In this case, the appropriate focusing control cannot be expected.